This invention relates to joining glass bodies along their sealing surfaces with an intermediate layer of sealing material to produce a composite article. It is particularly concerned with the production of a composite article which embodies a thermally sensitive element such as a metal conductor, a phosphor coating, or a reflecting film of a metal such as aluminum. While obviously broader in scope, the invention specifically provides a novel electric lamp construction of the type commonly known as a sealed beam lamp and widely used in vehicle headlamps.
Such lamps tend to operate at elevated temperatures. This factor, taken with the severe climatic conditions to which the lamp may be exposed in operation, requires a glass strongly resistant to thermal shock. Accordingly, it is customary to mold the reflector and lens components from a borosilicate glass having a thermal coefficient of expansion (0.degree.-300.degree. C.) in the range of 30 to 40.times.10.sup.-7 /.degree.C.
In current commercial assembly practice, the filament is precisely positioned within, and sealed into, the reflector member. That member is then aligned in opposed relationship to the lens member, the peripheral edges softened by flame heating, and the edges brought together and worked to form a direct fusion seal. It is generally necessary to bring the glass sealing edges to a temperature on the order of 1300.degree. C. to effect a reliable seal.
At the elevated temperatures required for flame sealing, there is a tendency for the glass envelope, and especially the reflector portion thereof, to warp. This requires an application of air pressure to restore the requisite contour to the sealed lamp. Meanwhile, the aluminum reflector film frequently becomes oxidized, thus reducing its efficiency. Finally, considerable care is necessary to minimize damage to the lamp filament during a flame sealing operation. It will of course be apparent that similar, and often more aggravated, problems arise in other sealing operations where thermally sensitive elements are involved, such as encasing electronic components in glass envelopes.
A factor of recent origin is the potential unavailability of gas as a fuel, at least during certain critical periods of high consumption. Thus, there is an obvious need for a reliable substitute sealing procedure, at least on a standby basis. Further, such substitute procedure should provide an overall diminished energy impact.
The principle of using a soft sealing glass as a replacement for a direct flame seal has been recognized for at least the past 30 years, as evidenced by U.S. Pat. No. 2,643,020, granted June 23, 1953 to R. H. Dalton. The stable glass seal of Dalton was followed by a thermally devitrified type glass seal as described generally in U.S. Pat. No. 2,889,952, granted June 9, 1959 to S. A. Claypoole. That type of seal has been widely used in cathode ray tube production for color television reception. However, the long temperature cycles of cathode ray tube production, which may be on the order of an hour, are detrimental to elements in sealed beam lamps even at the relatively moderate temperatures involved.
Further, cathode ray tube glasses normally have expansion coefficients on the order of 90 to 100.times.10.sup.-7 /.degree.C. Thus, while very satisfactory sealing glasses are available for use with glasses in this expansion range, the situation is quite different where, as in sealed beam lamps, parts with expansions in the 30 to 40.times.10.sup.-7 /.degree.C. range are involved.